The washer's go between the shield and the lens holder. I had 3 #6 washer's on all 3 of the points where the lens holder is screwed on. Currently on my Rx330 setup I have 3 on only the bottom 2 mounting points. Just start with 1 and go to 2 and so on until your happy.
Here's a pic of where you'll want to install them. I just editted VXCL's photo because I don't have my own with me at work.